1. Field of the Invention
The invention relates to a d.c. voltage supply circuit.
2. Description of the Related Art
There have been known d.c. voltage supply circuits comprising switch-mode power supply units which contain a series-circuit connected to the supply voltage and consisting of a switch and an inductor. The switch is controlled to alternatingly become conductive or nonconductive in order to charge the inductor in the conductive phase. In the nonconductive phase of the switch, the inductor is discharged to the load. The control of the switch is performed via a regulator circuit responsive to a current or voltage value of the series circuit. Switch-mode power supply units are advantageous because only relatively low inductances are required for generating d.c. voltage, and d.c. voltages higher than the supply voltage may be also produced.
Two types of switch-mode power supply units may be distinguished, namely the forward converter and the flyback converter, neither of which is well-suited to generate an auxiliary voltage for powering the regulator circuit. Basically, the auxiliary voltage may be obtained from the generated d.c. voltage. However, as a rule, the auxiliary voltage is much lower than the generated d.c. voltage. If the auxiliary voltage is obtained from the d.c. voltage by a voltage divider, this would entail high power losses, particularly if the d.c. voltage supply circuit is capable of generating voltages of different values. In place of a voltage divider, use could be made of a constant current source to generate at a resistor a defined voltage drop which may be used as an auxiliary voltage. In the case of such a circuit, although losses would be independent from the level of the generated d.c. voltage, a high expenditure would be required and the power dissipation would be still considerable.